A birefringent optical waveguide or fiber, such as a polarization-maintaining fiber that suppresses coupling or crosstalk between plural polarizations of waves, has been found to be advantageous. In particular, polarization maintaining fibers are useful in coupling laser diodes to waveguides that support only a single polarization, such as titanium indiffused waveguides in lithium niobate.
Several techniques are known for providing birefringent optical fibers. For example, see U.S. Pat. No. 4,395,270 to Blankenship et al. The '270 patent teaches a fiber that includes a cladding having two longitudinally-extending regions formed of a material having a thermal coefficient of expansion (TCE) that is different from that of the cladding. When the fiber is drawn, the longitudinally extending regions and the cladding regions therebetween shrink different amounts and cause a stress to be applied to the core of the fiber, which in turn promotes birefringence.
Various techniques for making an optical fiber having longitudinally extending regions have been proposed. For example, Blankenship et al., discussed above, teaches forming an optical fiber preform by providing a glass tube, inserting a core rod into the glass tube, inserting two stress rods having a particular TCE into the glass tube on opposing sides of the core rod and then filling the interstices with additional glass rods each having a different TCE than that of the stress rods. Optical fiber is then drawn from the preform. U.S. Pat. No. 4,561,871 to Berkey proposes another technique for forming a birefringent optical fiber. In particular, Berkey describes providing a preform of glass having a core and a cladding and forming a pair of longitudinally extending holes in the cladding. A pair of rods is formed from a material having a TCE different from that of the cladding and, thereafter, the rods are inserted into the holes in the cladding.
A photosensitive optical fiber has a refractive index that can be relatively permanently modified via exposure of the fiber to actinic radiation having a selected wavelength. Photosensitive optical fiber can be useful in many applications. For example, a photosensitive fiber may be selectively exposed to ultraviolet (UV) radiation to form an index grating, such as a Fiber Bragg Grating (FBG), in the core and/or the cladding of the optical fiber. An index grating can include spaced regions of the fiber wherein the index of refraction is different than the normal index of refraction of the fiber. An index grating may be written using various techniques known in the art, such as by using an amplitude mask, a phase mask, or by using interferometric techniques. An index grating may be used in a fiber coupled to a laser to stabilize the output wavelength of the laser. Also, an index grating may be employed with a circulator to select a particular wavelength out of multiple wavelengths propagating along an optical fiber. Further, index gratings may be used as sensors, as well as in many other applications, including providing memory storage.
It is desirable in certain circumstances that an optical fiber have an enhanced photosensitivity, such that the process of writing an index grating in the fiber via the selected exposure of the fiber to actinic radiation, which can take a significant period of time, be shortened. One example of a photosensitive fiber is described in U.S. Pat. No. 6,229,945 to Ainslie et al., which teaches that high concentrations of boron be used in a fiber to enhance the photosensitivity of the fiber. Boron, however, can have significant disadvantages. Germanium is known to be photosensitive in the absence of boron, but boron is useful in enhancing the photosensitivity of a germanium doped fiber.
Although highly photosensitive optical fibers are known in the art, as are highly birefringent optical fibers, Applicants are not aware that the art teaches how to successfully provide a useful optical fiber that provides a selected birefringence and a selected photosensitivity. Such a fiber would represent a welcome advance in the art and have many uses.
Accordingly, it is an object of the present invention to address one or more of the foregoing disadvantages or deficiencies of the prior art.